Diffusion of Ga into quartz (crystal SiO2) and bulk‐fused silica (amorphous SiO2) was studied by neutron activation analysis. Slow and fast diffusion of Ga was observed in bulk‐fused silica, while in quartz only slow diffusion was observed. The difference in the diffusion mechanisms is discussed on the basis of the difference in porosity of the two materials. In the bulk‐fused silica, which has a lower density of 2.20 g cm−3, there are two regions with high and low densities. The higher density region is composed of cristobalite, which has the maximum distance between atomic strings of (at most) 6 A˚. The lower density region is composed of a random network of Si and O atoms, and in the network there exist many microchannels, an average diameter and the real density of which are estimated to be 17 A˚ and 7.5×1011cm−2, respectively. These microchannels act as the high‐speed diffusion paths, while the cristobalite region is the slow diffusion path. In quartz which has a density of 2.65 g cm−3, the widest interatomic spacing perpendicular tocaxis is about 5 A˚ and only the slow (bulk) diffusion can occur. The discrepancy in the diffusion coefficients of Ga reported previously by Grove and Wagner can be explained by the present model.